1. Field of the Invention
The present invention relates to an optical isolator used for an optical communication, optical information processing, optical measurement and so forth.
2. Related Art
Optical isolators are used for optical amplifiers, semiconductor laser units and so forth.
These optical isolators comprise two of polarizers fixed to each other with a relative angle of about 45xc2x0 via a Faraday rotator inserted between them, which has a Faraday rotation angle of about 45xc2x0. They have an effect for passing through light of the forward direction but shielding light of the reverse direction.
Recently, smaller size, large-scale production and lower cost of these optical isolators are strongly desired. Accordingly, an optical isolator, for example, as disclosed in Japanese Patent Laid-open Publication No. 10-227996, have been proposed as a measure for answering the aforementioned demands, and such an isolator has been extensively developed and utilized.
As shown in FIG. 4(a), this optical isolator 20 comprises a rectangular parallelepiped optical element consisting of polarizers 22A and 22B and a Faraday rotator 23, and rectangular parallelepiped magnets 25A and 25B, and the element and magnets are bonded and fixed with an alloy solder, synthetic resin adhesive or the like on a tabular substrate 24. Further, FIG. 4(b) shows the case when the optical element and magnets are bonded with a bonding agent to the substrate, the bonded surfaces of both the element and magnets correspond to a whole area of the substrate surface. 22d represents each bonded surface of the polarizers, 23d represents the bonded surface of the Faraday rotator, and 25d represents each bonded surface of magnets.
Thus composed optical isolator seems to have advantages that it is easily positioned in a transmission polarizing direction when it is loaded inside an LD (laser diode) module, it can be loaded upon TEC (Thermoelectric Cooler), and it is easy to be downsized.
However, if both the optical element and magnets are disposed on one surface as described above, the following problems have occurred:
(1) Since both an optical element and magnets are bonded to one surface, magnetic fields of magnets in both sides affect the optical element (in terms of the attraction or repulsion), and thereby the optical element disposed in the center is distorted.
(2) Since magnetic field strength of a Faraday rotator disposed in each magnet end portion becomes lower contrary to that of the Faraday rotator disposed in each magnet center portion, the Faraday rotator can not obtain desired optical characteristics (of insertion losses in both a forward direction and reverse direction).
(3) It is difficult to position both an optical element and magnets.
(4) Desired optical characteristics can not be obtained due to the distortion generated when a rectangular parallelepiped optical element consisting of individual optical elements is bonded to a substrate.
(5) In order to avoid bonding distortion for an optical element, it has been known that the distortion can be relieved by previously bonding an optical element with a synthetic resin adhesive, especially silicone adhesive on a substrate. However, in this case, the sufficient strength for bonding between a substrate and optical element can not be obtained, and thereby there is a risk that the optical element is likely to be broken when it is handled.
Therefore, the present invention was accomplished in view of the aforementioned problems, and its object is to provide a reliable and low cost optical isolator that can bond and integrate an optical isolator element and permanent magnets on a substrate, can realize positional adjustment in assembly with high precision and high bonding strength, and can avoid bonding distortion for an optical element.
In order to solve the aforementioned problems, the present invention provides an optical isolator, wherein on one side of an optical element comprising at least one polarizer and at least one Faraday rotator, said optical element is bonded and fixed to a substrate in which a height of an optical element bonded portion suface has a level difference from a height of a peripheral portion surface.
As described above, if the optical isolator is bonded and fixed to a substrate in which the height of the optical element bonded portion surface has a level difference from the height of the peripheral portion surface, the distortion generated by a force due to absorption or repulsion affected by the magnet fields of the magnets, which is disposed on the peripheral portion of the substrate, for the optical element, which is disposed in the center of the substrate, can be relieved. Also, since the optical element can be disposed in the center of magnets in a height direction, a lack of the magnetic field strength for the Faraday rotator can be solved, and desired optical characteristics can be obtained. Further, by utilizing a level difference, there can be provided a reliable and low cost optical isolator that both the optical element and magnets can be positioned easily with high precision, and the optical element can be bonded and fixed with little adjustment.
Further, in order to solve the above-described problems, the present invention provides an optical isolator, wherein an optical element comprising at least one polarizer and at least one Faraday rotator, which are bonded to each light-transmitting surface, is bonded and fixed with a bonding agent to the substrate while the Faraday rotator in the optical element is not bonded to the substrate.
As described above, since only the Faraday rotator surface for the substrate in the optical element is not bonded to the substrate, the distortion for the optical element generated after it is bonded to the substrate can be solved, and desired optical characteristics can be obtained. Further, there can be provided with a reliable and low cost optical isolator that the optical element can realize positional adjustment with high precision, an can easily be bonded and fixed with sufficient high bonding strength.
In this case, a bond avoidance means can be provided so as not to be bonded the Faraday rotator in the optical element to the substrate.
According to the present invention, the optical isolator is suitable for being downsized and loaded inside an LD module. The present invention can provide a reliable and low cost optical isolator that a polarized surface of the optical element can be easily adjusted on a optimum position where its optical characteristics can be kept in good condition, the bonding strength for the substrate of the optical element is extremely high, and the optical isolator does not suffer a bonding stress, so that can obtain a large reverse direction insertion loss.